Apparatus for counting bank notes usually incorporates thickness measuring equipment in order to ensure that where two notes so adhere together as to pass through the apparatus as one note, they are not counted as one note because the thickness measuring equipment identifies them as thicker than a single note.
Conventionally bank note thickness measuring equipment comprises a mechanically complex mechanism in which notes are passed successively through rollers which are displaced from each other by the thickness of the passing notes. By their very nature, measuring as they do thicknesses of the region of 0.004" (0.10 mm) these mechanisms should be very precise, but their precision has led them to be complex and hence massive. They must operate at high speed and their massiveness leads to the generation of high mechanical inertia forces during the passage of notes through them. These forces take a relatively long time to be damped out in comparison with the time taken for the notes to pass through the mechanism. Traditionally these problems of complexity and high inertia have been accommodated by ensuring that one of two nip rollers through which the bank notes pass remains translationally fixed in a rigid datum frame, with respect to which the other nip roller is moved by a note passing between the two rollers. The rigid datum frame is a source of weight and cost. A source of error can be that the actual measurement taken is that of the movement of the movable roller and inaccuracies can be introduced due to the linkages involved. Examples of this type of prior equipment are described in British Patent specification No. 1497181, published on Jan. 5, 1978 in the name of De La Rue Crosfield Ltd. and European Patent specification No. A2 0080309 published on June 1, 1983 in the name of De La Rue Systems Ltd.
Generally it has not been possible to arrange more than one pair of nip rollers across the width of the note path because of the need to precisely guide the other movable roller.
Bank note thickness measuring equipment operating as a result of physical properties related to the thickness of the notes does exist. For instance one commonly used equipment employs the light absorption characteristic of bank notes, detecting the difference in light levels passing through one note or two notes. However such equipment is inherently unreliable in that dirty notes absorb markedly more light than clean notes.
The object of the present invention is the provision of improved thickness measuring equipment in particular though not exclusively for bank note counting apparatus.